The present invention relates generally to the control of temperature of an article. More particularly, the present invention relates to a system and method for the control of reticle temperature in lithography systems, especially in a vacuum environment.
In an electron or ion beam lithography system or a semiconductor exposure apparatus, an electron beam or ion beam projector directs electron or ion beams to a resist layer on a wafer substrate through a reticle which is typically placed on a reticle support or stage. The electron or ion beams are directed toward particular areas of the reticle to expose patterns on the reticle onto the wafer substrate. Thus, because the electron or ion beams of radiation are of relatively high energy, the areas of the reticle being exposed absorb power from the incident electron or ion beams and is heated thereby. Heat strain on the reticle caused by temperature changes impairs accuracy and may cause distortions and errors.
For example, when a 100 kV, 100 xcexcA electron beam is unblanked and directed at a reticle, a 2 xcexcm silicon membrane or stripe of the reticle absorbs approximately 200 mW from the electron beam. Given a coefficient of thermal expansion of 2.6 ppm/K for silicon, a 1xc2x0 K. rise over a 132 mm stripe length will cause approximately 343 nm of expansion. Such expansion may lead to error in the pattern exposed onto the wafer and reduce yield. To avoid such error and distortion, the temperature of the reticle is therefore preferably controlled to within a small fraction of a degree.
However, because the reticle is in a vacuum, controlling the temperature of the reticle silicon membrane presents a difficult challenge. In particular, heat transfer through convection is not available in a vacuum. Without convection, heat must be removed from the silicon by conduction and/or radiation.
In addition, removal of heat from the reticle by conduction is also difficult to achieve in a vacuum. In air, most of the heat transfer between, for example, two metal plates in contact with each other is actually transferred by convection across microscopic gaps between the metal plates with air serving as the fluid. The microscopic gaps generally result from surface roughness. Since the microscopic gaps are relatively small, the thermal conductivity is high and the overall thermal conductivity is usually determined by the material properties of the metal plates.
In a vacuum, heat transfer by conduction requires good thermal contact between two surfaces. To achieve good thermal contact between two surfaces, the clamping forces between the two surfaces must be very high. Alternatively, a compliant material or gasket may be utilized between the two surfaces. A third approach to overcome the contact thermal resistance problem in the case of the reticle heating in a vacuum is to provide a coolant in direct contact with the reticle.
These above-described approaches are undesirable for achieving reticle cooling particularly in view of the operating constraints of the lithography system. For example, reticles must be installed and removed quickly and repeatably on the reticle stage. Applying high clamping forces to the reticle would make the reticle installation and removal from the reticle stage time consuming, difficult and would likely not provide adequate repeatability. In addition, high clamping forces would likely create significant distortion. Installing gaskets would also not provide adequate repeatability and the gaskets are subject to wear and particulate generation. Attaching and disconnecting coolant sources and interconnections would also be very difficult and time consuming.
For the reasons set forth above, radiation has been explored as a method to cool the reticles or masks in a vacuum for both electron and ion lithography systems. For example, U.S. Pat. No. 4,916,322 entitled xe2x80x9cArrangement for Stabilizing an Irradiated Maskxe2x80x9d to Glavish et al., the entirety of which is incorporated herein by reference, discloses providing one or more cooling surfaces disposed adjacent the mask and the mask stage. The cooling surface surrounds an optical path of the beam in the field of view of the mask in the mask exposure station between the mask and the radiation source and/or behind the mask. As energy from the energy beam is transferred to the mask, the cooling surfaces compensate for the thermal energy transfer by transferring thermal energy by thermal radiation from the mask to the cooling surface. Such thermal energy compensation by the cooling surfaces is said to maintain the mask at approximately the chamber temperature during an irradiation. The cooling surface may be provided by a metal cooling tube which has a diameter larger than the mask such that the cooling tube does not block the optical path of the radiation source.
However, the cooling surface or tube disclosed by Glavish et al. is centrally place over the entire reticle, despite that the ion or electron beam is only focused on specific lines or areas of the reticle and does not uniformly heat the entire reticle at one time. Glavish et al. merely attempt to control the temperature of the reticle as a whole by cooling. Glavish et al. do not attempt to compensate for localized temperatures changes which may cause reticle distortion. Thus, an undesirable temperature gradient may nonetheless result.
U.S. Pat. No. 5,390,228 entitled xe2x80x9cMethod of and Apparatus for Stabilizing Shapes of Objects, Such as Optical Elements, as well as Exposure Apparatus Using Same and Method of Manufacturing Semiconductor Devicesxe2x80x9d to Niibe et al., the entirety of which is incorporated herein by reference, discloses determining a temperature distribution of a mask in a thermally stable state and controlling the temperature distribution of the mask being irradiated with radiation energy to be the same as the temperature distribution in the thermally stable state. The temperature distribution is controlled by providing a holder which holds as well as cools the mask and a heating means having resistance wires on a surface of the mask facing the direction of the incident beams to heat the surface of the mask.
The temperature control disclosed by Niibe et al. is of the entire reticle and because Niibe et al. utilize a reflective mask, Niibe et al. are not concerned with the localized heating of the reticle in an ion or electron beam system which can cause physical distortion of the reticle. Thus, Niibe et al. merely attempt to control the temperature of the reticle as a whole by cooling and heating.
It would thus be desirable to provide a method for reticle temperature control to reliably maintain the temperature of the reticle within a small fraction of a degree. It would also be desirable to improve temperature uniformity over the surface of the reticle. It would further be desirable to provide a method for reticle temperature control which is not time consuming, is simple to implement and provides good repeatability characteristics.
The present invention comprises a method for the control of the temperature of an article, particularly an article placed in a vacuum chamber and where the article is subjected to localized energy inputs. The method of the present invention comprises selectively applying irradiation to regions of the article to achieve and maintain temperature uniformity across the article. Since the application of radiation heat with radiation heat sources is non-contacting and obviates the need for physical contact or wires leading to the article, the temperature control apparatus can be relatively simple.
The system and method of the present invention may be utilized to control the temperature of a reticle in both electron and ion lithography systems or other systems. Radiant heat cycles are applied to control the temperature of the reticle geographically, depending upon the areas heated by the beams as a function of time and upon cycles such as wafer load cycle during which the reticle may experience a temperature decrease.
The method generally comprises applying initial heat after the reticle is initially loaded into the lithography system from an external environment, applying exposure heat when other reticles are being exposed and applying heat during the wafer load cycle when a new wafer is loaded and the electron or ion beam is blanked. The initial heat is only applied once when the reticle is initially loaded into the lithography system from the external environment
The present invention uniformly maintains the operating temperature of the reticle at a temperature slightly above the ambient temperature. When an incident electron or ion beam is directed at a region of the reticle, the localized temperature of that region will increase due to the energy of the beam. To equalize the localized temperature increase, radiant heating is applied selectively to certain other regions of the reticle in order to achieve and maintain temperature uniformity across the reticle. Achieving and maintaining temperature uniformity of the reticle is important for fabricating defect-free wafers, resulting in high yields.
The higher operating temperature of the reticle can be easily accounted for and compensated during design to ensure accuracy and result in a high yield. For example, the higher operating temperature of the reticle can be compensated by slightly lowering the ambient temperature, adjusting the design of the reticle and/or adjusting the application of the electron or ion beams in the lithography system.